1. Technical Field
The present invention relates to a wire electric discharge machining apparatus in which machining electric power is supplied between a wire electrode and a work piece so as to machine the work piece by discharge energy, and particularly to an improvement of such a wire electric discharge machining apparatus.
2. Background Art
FIG. 8 is a sectional view of a portion of a wire electric discharge machining apparatus disclosed in JP-B-4-25091, showing a part thereof in which machining is carried out. In FIG. 8, a pair of, that is, upper and lower, electrically conductive nozzle units 1 and 1xe2x80x2 are disposed to face each other symmetrically with respect to a work piece 2 which is fixed to a surface plate (not shown) by a retainer (not shown). Because the upper, and lower electrically conductive nozzle units 1 and 1xe2x80x2 are identical with each other in configuration and function, constituent elements of the lower electrically conductive nozzle unit 1xe2x80x2 will be referenced correspondingly to those of the upper electrically conductive nozzle unit 1 but with their reference numbers put with dashes (primes). Description will be therefore made below only as to the upper electrically conductive nozzle unit 1. In FIG. 8, the upper electrically conductive nozzle unit 1 is constituted by a nozzle holder 3, a nozzle 4, a box nut 5, a spring 6, an O-ring 7, a machining liquid supply conduit 8, a wire electrode 10, a guide die 9 for guiding the wire electrode 10, a die holder 11, a conductive element mounting bracket 12 attached to the forward end of the nozzle 4, an insulation member 13, a conductive element 14, a plate 15, a conductive pin 16, and a guide roller 17. The conductive element mounting bracket 12 is constituted by a conductive element housing 12a shaped like a ring or divided into a plurality of segments in the conductive element mounting bracket 12, and a conduit 12b formed in the conductive element mounting bracket 12 to supply machining liquid to the conductive element housing 12a. The insulation member 13 covers the inside of the conductive element housing 12a. The whole of the conductive element 14 is shaped like a ring or divided into a plurality of segments each having a predetermined angle. The plate 15 receives the conductive element 14 which is slidable in the conductive element mounting bracket 12, and allows the forward end portion of the conductive element 14 to be exposed from the plate 15. The plate 15 has liquid draining holes 15a. The conductive pin 16 supplies electric power to the wire electrode 10. The guide roller 17 guides the wire electrode 10.
The nozzle holder 3 is constituted by a mounting flange 3a, a nozzle chamber 3b, a bore 3c for inserting the wire electrode 10, a hole 3d to which a machining liquid supply conduit 8 is attached, and a thread portion 3e. The die holder 11 is fixed to the nozzle holder 3, and further, the spring 6 and the box nut 5 are attached thereto in the order. On the other hand, although the nozzle 4 is slidable upward and downward in the nozzle chamber 3b, the nozzle 4 is halted at a movable end separated from the work piece 2 by a restoring force of the compressed spring 6 when the nozzle 4 is not in a machining state.
Further, the nozzle 4 has a small hole 4a, as a machining liquid discharging port, at one end of the nozzle 4. The wire electrode 10 fed from a feeder (not shown) is pulled into the inside of the electrically conductive nozzle unit 1 through the wire electrode inserting bore 3c of the nozzle holder 3, and pulled out from the small hole 4a of the nozzle 4 via the conductive pin 16 and the guide die 9. Then, the wire electrode 10 passes through the inside of the lower electrically conductive nozzle unit 1xe2x80x2, and finally, is recovered by a recovery apparatus (not shown).
The conductive element 14 is received slidably in the conductive element housing 12a, and designed to always come into contact with the surface of the work piece 2 during machining by the function of the hydraulic pressure of the machining liquid. Further, an output terminal of a machining power supply (not shown) is connected to an electrically conductive member which is integrally formed with the conductive element 14 extruding from the conductive element mounting bracket 12 toward a side opposite to the plate 15. Further, for example, a capacitive discharge circuit with a small stray inductance is formed in such a manner that one terminal of a machining gap capacitor (not shown) is connected to the above-mentioned electrically conductive member while the other terminal of the machining gap capacitor is connected to a power supply terminal to the conductive pin 16.
The wire electric discharge machining apparatus configured thus can obtain larger discharge current amplitude so as to improve the machining speed.
However, in the conventional wire electric discharge machining apparatus described above, the conductive element 14 for supplying electric power to the work piece 2 is received slidably in the conductive element housing 12a, and always pressed against the surface of the work piece 2 during machining by the function of the hydraulic pressure of the machining liquid. Accordingly, particularly in finishing machining, there is a problem that the machining accuracy such as surface roughness and dimensional accuracy or the like is lowered.
Further, in the case where the electric power is supplied to the work piece 2 not from the conductive element 14 but from the surface plate (not shown) or the like supporting the work piece 2, there is another problem that the machining speed is decreased.
The present invention was achieved in order to solve the above problems. Therefore, an object of the present invention is to provide a wire electric discharge machining apparatus which is high in productivity and which can prevent the machining accuracy such as surface roughness and dimensional accuracy or the like from being lowered.
According to an aspect of the present invention, there is provided a wire electric discharge machining apparatus in which machining liquid is interposed between a wire electrode and a work piece and the work piece is machined by discharge energy, the wire electric discharge machining apparatus being constituted by: electrically conductive means supported so that a state of the electrically conductive means is switched over between a contact state where the electrically conductive means comes into contact with the work piece and a non-contact state where the electrically conductive means does not come into contact with the work piece; drag generating means for generating drag acting on the electrically conductive means in a direction to make the electrically conductive means move away from the work piece; machining liquid pressure controlling means for changing and controlling a hydraulic pressure of the machining liquid; and electrical conduction switch-over means for switching over power supply to the work piece between a direction from the electrically conductive means to means other than the electrically conductive means and another direction from the means other than the electrically conductive means to the electrically conductive means, wherein the hydraulic pressure of the machining liquid during electric discharge machining is changed and controlled by the machining liquid pressure controlling means so that the state of the electrically conductive means is switched over between the contact state where the electrically conductive means comes into contact with the work piece and the non-contact state where the electrically conductive means does not come into contact with the work piece in accordance with a desired machining speed and desired machining accuracy to thereby carry out machining.
Further, according to another aspect of the present invention, there is provided a wire electric discharge machining apparatus in which machining liquid is interposed between a wire electrode and a work piece and the work piece is machined by discharge energy, the wire electric discharge machining apparatus comprising: electrically conductive means supported so that a state of the electrically conductive means is switched over between a contact state where the electrically conductive means comes into contact with the work piece and a non-contact state where the electrically conductive means does not come into contact with the work piece; drag generating means for generating drag acting on the electrically conductive means in a direction to make the electrically conductive means move away from the work piece; and electrical conduction switch-over means for switching over power supply to the work piece between a direction from the electrically conductive means to means other than the electrically conductive means and another direction from the means other than the electrically conductive means to the electrically conductive means, wherein a force generated by the drag generating means is changed and controlled so that the state of the electrically conductive means is switched over between the contact state where the electrically conductive means comes into contact with the work piece and the non-contact state where the electrically conductive means does not come into contact with the work piece in accordance with a desired machining speed and desired machining accuracy to thereby carry out machining.
Further, preferably, the electrically conductive means is a nozzle for spraying and supplying the machining liquid.
Since the present invention is configured as described above, there is an effect that both machining speed and machining accuracy can be improved.